1. Field of the Invention
This invention relates generally to cookware and, more particularly, to cookware having a stick resistant coating applied thereto.
Heretofore, it has been common to apply a lubricative layer of PTFE (polytetrafluoroethylene) to the cooking surface of metal cookware to provide, at least initially, a relatively stick-free surface. Over time, the PTFE loses lubricity and becomes less stick-free or somewhat stick resistant. The PTFE surface is somewhat objectionable because much softer than the metal substrate and is easily scratched and otherwise marred by metal spatulas, forks and other kitchen utensils used for cooking. Once scratched or otherwise marred, the PTFE surface can be chipped off and generally loses its original utility as a stick-free surface.
2. Description of the Related Art
More recently, attempts have been made, as disclosed in UK Patent Application GB 2,277,466A to Tsai, to increase the scratch resistance of PTFE. Other work in the area aimed at improving the hardness and durability of stick resistant coatings for cookware is embodied in U.S. Pat. No. 5,447,803 to Nagaoka et al., which discloses a coating of titanium nitride to steel fry pans. The TiN coating is applied by physical vapor deposition (PVD) or by chemical vapor deposition (CVD) and then heat treated in an atmosphere consisting of oxygen and nitrogen in a controlled ratio to form a protective layer of titanium oxide on the surface of the titanium nitride layer. The titanium nitride layer is said to be hard and, in addition, produces a pleasing golden color to improve the appearance of the fry pan.
Zirconium nitride is a known ceramic coating which has been applied to enhance hardness and wear resistance to various industrial goods such as drill bits, lathe bits and injection molds.
I have discovered that zirconium nitride provides an excellent long-lasting, stick resistant coating for cookware when the substrate metal is properly selected and prepared. In addition, zirconium nitride does not require a separate treatment step to produce an oxide surface as is necessary with prior titanium nitride surfaces. Further, the zirconium nitride coating of my invention provides a pleasing golden color on the cookware which does not discolor when exposed to direct flame, which occurs with titanium nitride coatings. The zirconium nitride coating of the present invention is extremely hard and abrasion resistant, which provides a durable, long-lasting, stick resistant cooking surface, thus making it particularly suitable for restaurant and institutional kitchens. The pleasing gold tone color of the zirconium nitride coating, coupled with its stick resistant properties on the cookware of the present invention, likewise, makes it suitable for the houseware consumer market.
The present invention is directed to a cooking or baking surface, hereinafter referred to collectively as xe2x80x9ccook surfacexe2x80x9d, having a stick resistant coating of a ceramic nitride material, preferably zirconium nitride, thereon. The invention also relates to a method of making the coated cook surface, as well as to the resultant cooking or baking vessels made by the method.
Briefly stated, the cook surface of the invention comprises a hard substrate metal such as stainless steel, carbon steel, titanium or the like, which is buffed to a high luster finish of less than 20 micro inches, preferably in the range of 9-12 micro inches, and still more preferably about 2-6 micro inches. The buffed surface of the substrate metal is cleaned and then coated with the preferred material, zirconium nitride. There is no need to heat treat the applied coating to produce a protective oxide layer, since zirconium nitride is self-passivating in the atmosphere and, thus, forms a protective layer of zirconium oxide without any special treatment.
The substrate metal of the cooking vessel can be a single layered material or it may be bonded as a clad composite to layers of various other metals such as a conductive core material, for example, copper or aluminum, to promote thermal conductivity. Magnetic layers of ferritic stainless steels may also be included in the composite to make the vessel compatible with induction heating ranges.
When the substrate metal is part of a clad composite of various metal layers, as described above, the zirconium nitride coating is preferably applied by the cathodic arc method known in the art as the physical vapor deposition method (PVD). This method uses a vacuum chamber pressure on the order of 10xe2x88x925 Torr at a temperature of about 500xc2x0-900xc2x0 F. This temperature range promotes good coating adhesion while staying below the temperature at which stainless steel and aluminum (forming the clad composite) separate.
According to a preferred method according to the invention, when making deep drawn cooking vessels, such as pots and certain pans, it is difficult to buff or polish the entire interior surface to a bright luster finish. In such circumstances, a presently preferred method of the invention comprises the step of forming a flat blank of the substrate metal, in a clad composite form, if applicable, then buffing the cooking surface of the flat blank to a high luster finish and then drawing the thus-polished blank to a desired shape of the finished vessel. The shaped vessel is then cleaned and the buffed surface is coated with the zirconium nitride material to provide a stick resistant surface.
These features, as well as other advantages and attributes of the invention, will become more readily apparent when reference is made to the drawings when taken with the following detailed description.